1. Field of the Invention
This invention pertains generally to synthesis schemes and methods for producing silicon based thin film nanostructures and materials, and more particularly to compositions and methods for synthesis of silicon-based materials using liquid hydrosilane(s).
2. Description of Related Art
Silicon is one of the most abundant materials available in the earth in the form of silicates. Pure elemental silicon is an indirect-band gap semiconductor that is widely used in applications ranging from photovoltaic to microelectronic devices. Despite the arrival of several direct band-gap compound semiconductors, Si remains the material of choice due to its ready availability and simple processing. Bulk Si finds application in technologies such as c-Si solar cells, but Si in its thin film form is more widely used.
Thin films of Si are generally obtained by cracking or decomposing gaseous silicon precursors such as mono, di and tri silanes (SiH4, Si2H6, Si3H8) or silicon tetrachloride/fluoride with a suitable reducing atmosphere at elevated temperatures. The addition of plasma to CVD helps to decrease the processing temperature and increase the deposition rate. Silicon in thin film form can be deposited as amorphous, polycrystalline, nano-crystalline or in mixed phases depending on the process parameters that are used. Alternately Si thin films can be obtained from physical vapor deposition methods such as magnetron sputtering, cathodic arc, etc. However, the lower deposition rate associated with these processes limits their application. Due to the lower deposition efficiencies associated with the gaseous silanes (10-20% in PECVD with SiH4), large volumes of gases used in the manufacturing processes are stored in cylinders.
Silane is also extremely pyrophoric and ignites spontaneously in air even at concentrations as low as 2-4%. When used in combination with other gas(es) such as hydrogen, oxygen, nitrogen or ammonia, the hazard level increases. Such hazards associated with silanes in gaseous form demand complicated gas-handling systems with advanced safety features to reduce the hazard of fire and explosion.
Growing consumer demand for electronic products such as thin film transistor (TFT) based flat panel displays, Photovoltaic cells, etc., drives the demand for alternate processes and Si sources for silicon thin films to further decrease the cost of production. State-of-the-art manufacturing technology for Si-based electronic devices relies on vacuum processes which are difficult to implement with roll-to-roll production techniques. Low conversion efficiencies of SiH4 to Si thin films and the difficulties associated with handling pyrophoric silane gases are other factors increasing the cost of manufacturing with gaseous silanes. High order silane gases such as disilane (Si2H6), trisilanes (Si3H8) and tetrasilanes (Si4H10) are shown to improve deposition efficiencies, but the issues with gas handling still remain a challenge.
Liquid silane sources such as cyclopentasilane (Si5H10, CPS) and cyclohexasilane (Si6H12, CHS) provide an opportunity to deposit thin films of Si at ambient conditions with solution based processes such as spin coating, spray coating etc., which can be cost-effective for large scale production of electronics. Liquid hydrosilanes can be used as the precursor for silicon containing films and materials using chemical vapor deposition. Liquid hydrosilanes are easier to handle than gaseous silanes. These liquid silane precursors have been shown to produce films at higher deposition rates and relatively low temperatures compared with other gaseous silanes. However, liquid hydrosilanes may have lower-vapor pressure resulting in difficult vaporization and transportation in the vapor phase. In addition, prolonged exposure to heat and light radiation initiates polymerization in liquid silane, further reducing its vapor-pressure and the flow rate of the vapor. This can lead to large inconsistencies in the deposition process over time.
Several scientific challenges in utilizing the wet-chemical processes that govern the growth of Si thin films remain. For example, the extent of UV-induced polymerization, viscosity, and wettability of liquid silanes on different surfaces are parameters known to play a vital role in solution based processing. One difficulty observed with the use of liquid silanes is feed line clogging during the transportation neopentasilane vapor to a CVD reactor using standard techniques. Accordingly, there is a need for an alternative to conventional methods for producing silicon thin films that use hazardous gases as well as alternatives to the standard vacuum CVD/PECVD and spin coating processes. The present invention satisfies these needs, as well as others and is a significant improvement in the art.